1. Field of the Invention
This invention relates to a metal gasket used for sealing a portion between a cylinder head and a cylinder block of a multiple-cylinder engine.
2. Description of the Prior Art
In a multiple-cylinder engine, a cylinder head and a cylinder block are coupled with each other by bolts, and a metal gasket is sandwiched between the cylinder head and the cylinder block to prevent leak of a gas from their fitting surfaces. A metal gasket includes cylinder bore holes 2 bored in an elastic metal plate 1 and a bead 3 formed around the periphery of each hole 2 as shown in FIG. 1. In this metal gasket, the beads 3 define the seal portion on the fitting surfaces by the clamping force of the bolts, and exhibit an effective seal function.
Recently, a metal gasket has been developed wherein the respective beads 3 join with one another at a junction 4 between two adjacent holes 2 in such a manner as to define a common bead 5 between the holes 2 (e.g. U.S. Pat. No. 4,815,750 (corresponding to Japanese Patent Laid-Open No. 210465/1988) and Japanese Utility Model Publication No. 86878/1989). Such a metal gasket will be explained with reference to FIGS. 10 and 11. In the drawings which will appear, like reference numerals will be used to identify like constituents of the metal gasket. FIG. 10 is an enlarged explanatory view showing an example of the junction in a conventional metal gasket.
In FIG. 10, each of the two beads 3 is shaped into an arc along the hole 2 at the common bead 5 between the holes 2. A two-dot-chain line represents the center line 6 of the bead, and the center lines 6 of the two beads 3 are in contact with each other at the center of the common bead 5 between the junctions 4. Since the beads 3 have such a shape, the bead width L.sub.1, L.sub.2 of the non-junction (L.sub.1 =L.sub.2) is substantially equal to the bead width L.sub.3 at the center of the common bead 5. In contrast, the bead width L.sub.4 at the junction 4 at which the beads 3 join with each other is about twice the bead width L.sub.1, L.sub.2 of the non-junction and the bead width L.sub.3 at the common bead 5.
As a result, as can be understood from a face-to-face pressure (kgf/mm.sup.2) of the bead shown in the graph of FIG. 10 (a linear pressure (kgf/mm) in terms of an analytical value by a finite-element method), the face-to-face pressure drops drastically in the vicinity of a portion at which the bead width greatly changes, that is, near the junction position (regions X and Y encompassed by circles). However, the face-to-face pressure is the reaction that occurs in the beads formed along the holes of the metal gasket when the metal gasket is fitted under the compressed state between the cylinder head and the cylinder block. The drop of the face-to-face pressure is particularly remarkable at the portion (region X) near the center of the common bead 5. This drop of the face-to-face pressure is believed to result from a small spring constant at the junction 4. In other words, since the ratio of the bead width with respect to the bead height becomes drastically great at the junction position in comparison with the non-junction position, the spring constant becomes small at the junction 4 and for this reason, it is believed that the drop of the face-to-face pressure in the vicinity of the junction occurs. As the drop of the face-to-face pressure is remarkable at the junction 4 in the metal gasket shown in FIG. 10 as described above, sufficient seal performance cannot be secured because the face-to-face pressure of the bead affects seal performance.
FIG. 11 is an enlarged explanatory view showing another example of the junction of the conventional metal gasket. The metal gasket shown in FIG. 11 is of an improved type of the metal gasket shown in FIG. 10. In this metal gasket, the beads 3 are formed on the elastic metal plate 1 to perform the sealing function. The holes 2 are bored in the elastic metal plate 1, the bead 3 is so formed as to encompass each hole 2, and the beads 3 join with one another between the adjacent holes 2 and have a linear portion 7 at the center of the common bead 5. In this metal gasket, the bead width of the non-junctions is made progressively smaller towards the junction 4 so that the bead width of the junction 4 of the beads 3 for sealing of each seal portion provided to the elastic metal plate 1 becomes substantially equal to the bead width of the non-junction. In other words, the bead width L.sub.4 of the junction 4 is so shaped as to be substantially equal to the bead width L.sub.1, L.sub.2 (L.sub.1 =L.sub.2) of the non-junction, and the bead width L.sub. 4 of the junction 4 and the bead width L.sub.1, L.sub.2 of the non-junction are shaped into the same size so as to increase the spring constant at the junction 4, to avoid the problem of the drop of the seal pressure of the junction 4 and to prevent the face-to-face pressure from becoming non-uniform. As shown in the drawing, this metal gasket certainly has a large spring constant at the junction 4 and the face-to-face pressure does not drop when only the junction 4 is taken into consideration.
Nonetheless, according to analytical calculation by the finite-element method, the drop of the face-to-face pressure P.sub.L of this metal gasket is remarkably great at the boundary between the junction 4 and the non-junction as shown in FIG. 11 and is about 1/2 of the face-to-face pressure P.sub.M of the non-junction. Further, since the spring constant of the junction 4 is increased, the face-to-face pressure P.sub.MAX of the junction 4 scores an extremely high value, and non-uniformily of the face-to-face pressure is induced, conversely. Since this metal gasket has an unbalanced face-to-face pressure state as described above, its seal performance is not reasonably high, and if the spring constant and the face-to-face pressure are increased, undesirable problems with durability such as buckling and cracks will occur.
Therefore, the shape of the beads of the metal gasket shown in FIG. 11 will be examined. Although the bead width L.sub.4 of the junction 4 is the same as the bead width L.sub.1, L.sub.2 of the non-junction, two center lines 6 of the bead exist in the junction 4 as represented by a two-dot-chain line. Needless to say, the height of the bead 3 does not vary between the junction 4 and the non-junction. Therefore, the sectional shape of the beads 3 at the junction 4 describes two sharp humps in such a way that the bead width of the non-junction decreases progressively towards the junction 4. Since the bead itself is originally very thin and moreover, since the bead width is contracted at the junction 4, the sectional shape of the bead 3 changes complicatedly and drastically at the junction 4. For this reason, the face-to-face pressure becomes extremely great at such a junction 4 and as a reaction, the face-to-face pressure drops drastically on both sides of the junction 4. Moreover, machining of the junction 4 becomes extremely difficult. Among others, a mold for producing the bead 3 having such a sectional shape which changes in a complicated way is extremely difficult to produce, and practical machining is almost impossible because the production cost is very high.